1. Field
This invention relates generally to Asymmetric Digital Subscriber Line (ADSL) modems, and more particularly to a method of adaptive hybrid selection during ADSL modem training.
2. Description of the Prior Art
The hybrid is an analog electrical network (part of the analog front end-AFE) in the modem that is responsible for 2 to 4 wire conversion i.e., the hybrid allows the transmit signal from one pair of the 4-wire side (transmit) to pass directly on to the 2-wire side (subscriber loop) and at the same time suppresses the transmit signal component on the other pair of the 4-wire side (receive). Ideally, the hybrid should completely suppress the transmit energy on the receive wire pair. However, in reality since the line impedance changes from one loop to the next, the hybrid can only be designed to match an “average” loop impedance. In many cases, with bridge taps, HPNA devices and other devices on the line, the loop impedance can change substantially from the nominal value. In these cases, the hybrid is mismatched to the line and the transmit energy can completely dominate the receive signal and receive performance suffers. In order to reduce the effect of hybrid mismatch on receive performance, often multiple hybrids are designed into the modem. The important task then becomes one of selecting the hybrid that maximizes receive performance.
It would therefore be desirable and advantageous in view of the foregoing to provide a methodology for adaptive hybrid selection based on the estimated noise floor during regular ADSL modem initialization.